1. Field of the Invention
This invention relates generally to a method and apparatus for reducing jet engine noises of supersonic jet aircraft or subsonic jet aircraft.
2. Background Discussion
Supersonic and subsonic jet aircraft generate noises from several sources, such as engines, structures, and internal equipment. Jet engines have been recognized as a major noise source, generating noises that include inlet noise produced by inlet turbines, noise from bypass ducts, and noise produced by the exiting stream from the main engine exhaust nozzle. In general, the noise produced by a supersonic engine becomes more severe than a subsonic engine due to the high speed of the streams at the main exhaust nozzle and the presence of shock associated noise.
The intense noise radiation generated by jet aircraft capable of supersonic travel poses a direct threat to communities surrounding airports and persons in close proximity to these jet aircraft, such as on a jet aircraft carrier deck. Severe restrictions have been placed on these jet aircraft, limiting their travel in the supersonic region to so called supersonic corridors and over-water travel.
There has been a long-felt need to reduce noise associated with supersonic and subsonic jet aircraft. Efforts to reduce engine noise started as early as the 1950s. Despite many years of research and development, and notwithstanding noise reduction techniques, supersonic jet aircraft still suffer from intense noise generation without fully effective means of controlling the noise.
A number of approaches to reduce subsonic engine noise have been proposed and adopted by the aviation industry. For example, one approach includes the use of a passive device known as a lobe type mixing nozzle disposed at the exhaust of the engine. This nozzle forces the ambient air to mix at a faster rate with the exhaust stream, increasing the turbulent mixing rate and reducing the overall sound pressure level of the engine (OASPL). The main drawback to this type of nozzle is its heavy impact on the thrust of the engine due to the reduction in the exit area occupied by the device. An improved approach includes the use of a Chevron type nozzle.
Turbofan engines, which have been in operation for quite some time, were designed for efficient engine for transonic flight. Although not designed to reduce noise, some noise reduction is an unintended result.
Efforts to reduce the engine noise of supersonic jet aircraft have been attempted by many artisans in the field of endeavor for a long time. These techniques may be classified as “passive techniques” and “active techniques.”
Passive techniques apply additional structures to engines to change exhaust flow characteristics. For example, a simple structure, a flexible filament disposed along the centerline of a supersonic jet engine, has been tested for its effects on reducing engine noise. However, the flexible filament is structurally unsound. A more complex structure, an x-shaped “wire device,” has been located downstream of a supersonic jet engine, significantly modifying the stream structure of the engine. The usually-observed shock cell structure in other jet engines seems to be destroyed downstream of the device, leading to the reduction of all shock-associated noises. The x-shaped wire device, however, significantly alters the flight characteristics of a jet aircraft and dramatically increases the drag.
Active techniques mainly use microjet ports to inject either liquid or gaseous masses into streams exiting from jet engines. While the results of injecting liquid masses are promising, the practicality of liquid injection renders it severely limited as it requires the transport of a liquid source. Common applications of aqueous injection are seen in the noise reduction of launch vehicles, where the source needs not to be transported. Gaseous injection is a more practical approach to fluidic injection as the flow medium could be sourced from the compressor of the jet engine. Thus far, current techniques that inject gaseous masses still have unsolved issues when used in supersonic jet engines.